tg.sh: completely ignore hooksPath until setup_hook
The current state of core.hooksPath is completely irrelevant (and
should be left totally unmolested) unless setup_hook gets called.
Therefore delay all inspection and setup of git_hooks_dir until
such time as setup_hook actually gets executed. This means that
"read-only" tg commands that have no need to touch the repository
will never need to bother with the state of core.hooksPath at all.
Validate this with additional tests.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>